Can concrete be mixed with expansive agent?

The application of concrete has promoted the rapid development of admixtures and achieved considerable technical, economic and social benefits. 

Concrete admixtures are indispensable for key projects of high-rise buildings, and admixtures have become the fifth indispensable main component in concrete construction. 

According to incomplete statistics, the annual consumption of Chinese and foreign admixtures in commercial concrete has reached millions of tons. 

Because some construction technicians do not have enough understanding and application technology of admixtures, not all of them can achieve better results. In concrete construction, due to improper debugging and application of concrete, admixtures are suspected at first. 

What is concrete expansive agent? 

Expansive agent is a kind of admixture which can produce volume expansion under limited conditions to compensate all kinds of shrinkage of concrete, improve the compactness of concrete and improve the crack resistance and impermeability of concrete. 

For a long time, the author has seen a lot of successful examples applied to this product. 

But recently, there are more and more engineering accidents after the application of this product, which makes many engineers doubt the application effect of expansive agent, and even gives rise to the view that "concrete will not crack without expansive agent, the more it is used, the more it will crack." 

The above concepts also appear in the expositions of some experts and scholars. 

Whether expansive agent can not be mixed in commercial concrete, through accident analysis, the author thinks that it is one-sided that commercial concrete can be used unconditionally or not. 

First of all, the scope of application of expansive agent, through the analysis of the action mechanism of this product, the author thinks that it is not suitable to be mixed in concrete with high strength and other low water binder. 

This is because the lower water consumption of concrete does not guarantee that there is enough water to effectively produce volume expansion. 

Even if the watering and curing is strengthened, because the concrete with low water-binder ratio is relatively dense and impermeable, only a small amount of water can infiltrate into the concrete, which still can not meet the water demand of expansion. 

Secondly, due to the high water absorption of expansive agent, the cement particles in concrete can not be fully hydrated due to lack of water, which affects the strength of concrete. 

There are many kinds of expansion agents in our country, and the water absorption and expansion periods of these products are quite different, such as the setting time of cement is relatively late, but the expansion time of the expansion agents used is too fast, which makes the expansion agents ineffective. These expansion agents only play the role of admixtures on concrete. 

The hydration and water absorption expansion of expansive agent needs to supplement a large amount of water, and strengthening watering and curing is the first condition for expansive agent to give full play to expansion. 

According to the relevant codes, the watering and curing period of concrete mixed with expansive agent takes more than 14 days, but in the actual construction, more than 80% of the concrete construction can not guarantee such a long time of watering and curing, which affects the use effect. 

It must be pointed out that concrete mixed with expansive agent, like ordinary concrete, will produce its own volume shrinkage under dry conditions. 

If the expansion agent is restored to a wet environment, the expansion agent will still expand again. 

These shrinkage can also cause some cracks in the concrete, and the expansion cracks will be re-closed. 

Therefore, a small amount of cracks in the drying period after the addition of expansive agent has nothing to do with the product quality and will not affect the performance of concrete. 

In commercial concrete, expansive agent is completely suitable for ordinary concrete, waterproof concrete and hydraulic mass concrete below C40, but it is not suitable for C40 or above concrete. 

When mixing expansive agent in commercial concrete, we must fully understand the product performance, adopt appropriate content, and strengthen watering and curing, which can achieve certain technical results.

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